diff --git a/src/wp-includes/kses.php b/src/wp-includes/kses.php
index 2be0b5e14c3cf..7776de59d55d6 100644
--- a/src/wp-includes/kses.php
+++ b/src/wp-includes/kses.php
@@ -2229,7 +2229,7 @@ function kses_init() {
  * @since 5.7.1 Added support for `object-position`.
  * @since 5.8.0 Added support for `calc()` and `var()` values.
  * @since 6.1.0 Added support for `min()`, `max()`, `minmax()`, `clamp()`,
- *              and nested `var()` values.
+ *              nested `var()` values, and assigning values to CSS variables.
  *              Added support for `gap`, `column-gap`, `row-gap`, and `flex-wrap`.
  *              Extended `margin-*` and `padding-*` support for logical properties.
  *
@@ -2391,6 +2391,9 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'object-position',
 			'overflow',
 			'vertical-align',
+
+			// Custom CSS properties.
+			'--*',
 		)
 	);
 
@@ -2436,6 +2439,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 		$found           = false;
 		$url_attr        = false;
 		$gradient_attr   = false;
+		$is_custom_var   = false;
 
 		if ( strpos( $css_item, ':' ) === false ) {
 			$found = true;
@@ -2443,11 +2447,23 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			$parts        = explode( ':', $css_item, 2 );
 			$css_selector = trim( $parts[0] );
 
+			// Allow assigning values to CSS variables.
+			if ( in_array( '--*', $allowed_attr, true ) && preg_match( '/^--[a-zA-Z0-9-_]+$/', $css_selector ) ) {
+				$allowed_attr[] = $css_selector;
+				$is_custom_var  = true;
+			}
+
 			if ( in_array( $css_selector, $allowed_attr, true ) ) {
 				$found         = true;
 				$url_attr      = in_array( $css_selector, $css_url_data_types, true );
 				$gradient_attr = in_array( $css_selector, $css_gradient_data_types, true );
 			}
+
+			if ( $is_custom_var ) {
+				$css_value     = trim( $parts[1] );
+				$url_attr      = str_starts_with( $css_value, 'url(' );
+				$gradient_attr = str_contains( $css_value, '-gradient(' );
+			}
 		}
 
 		if ( $found && $url_attr ) {
diff --git a/tests/phpunit/tests/kses.php b/tests/phpunit/tests/kses.php
index 274c5aaf7be06..5871f0fe3397f 100644
--- a/tests/phpunit/tests/kses.php
+++ b/tests/phpunit/tests/kses.php
@@ -1247,6 +1247,23 @@ public function data_test_safecss_filter_attr() {
 				'css'      => 'margin-block-start: 1px;margin-block-end: 2px;margin-inline-start: 3px;margin-inline-end: 4px;padding-block-start: 1px;padding-block-end: 2px;padding-inline-start: 3px;padding-inline-end: 4px',
 				'expected' => 'margin-block-start: 1px;margin-block-end: 2px;margin-inline-start: 3px;margin-inline-end: 4px;padding-block-start: 1px;padding-block-end: 2px;padding-inline-start: 3px;padding-inline-end: 4px',
 			),
+			// Assigning values to CSS variables introduced in 6.1.
+			array(
+				'css'      => '--wp--medium-width: 100px; --var_with_underscores: #cccccc;',
+				'expected' => '--wp--medium-width: 100px;--var_with_underscores: #cccccc',
+			),
+			array(
+				'css'      => '--miXeD-CAse: red; --with-numbers-3_56: red; --with-url-value: url("foo.jpg");',
+				'expected' => '--miXeD-CAse: red;--with-numbers-3_56: red;--with-url-value: url("foo.jpg")',
+			),
+			array(
+				'css'      => '--with-gradient: repeating-linear-gradient(135deg,rgba(6,147,227,1) 0%,rgb(155,81,224) 100%);',
+				'expected' => '--with-gradient: repeating-linear-gradient(135deg,rgba(6,147,227,1) 0%,rgb(155,81,224) 100%)',
+			),
+			array(
+				'css'      => '--?><.%-not-allowed: red;',
+				'expected' => '',
+			),
 		);
 	}